failed Zs

[nicsin02]

Hi guys, I am currently testing a large hotel (over 300 rooms) it is only the 4th level we have to test this year (65 rooms) each room has its only 5-way d/b consisting of a 32 & 6 Rcbo type B, each of these d/b's are fed from different larger d/bs installed in various fire escapes along the corridor, I have found that these small d/b's in the rooms Zdb's are getting quite high the further away they are getting from their supply d/b some of them even going as high as 1ohms therefore making the Zs of the 32a circuit far too high, ie 1.76. Ohms etc.......

1. Is a mac Zs the same on a 60898 as a 61009?

2. If the max Zs test fails will they use the fact it is protected by an RCD so Zs is mute or is this Rcbo merely used as additional protection not fault protection?

Thanks guys

 

[Sintra]

The max permissible Zs on a 61009 is 1667 ohms although on a TN system they will be used as additional protection therefore IMO the maximum permissible Zs of the equivilant 60898 should be met.

 

[telectrix]

one solution might be to reduce the rating of the MCB to 25A or 20A. in a hotel, i would imagime that the current demand in each room is pretty small.

 

[Engineer54]

In circumstances such as you find yourself in, the Zs of a circuit MCB protective device is the value you follow and not the Zs value an RCD brings. This is a hotel, which brings additional hazards such as fire risks etc!! Also the installations within each of these rooms, will be used by paying customers, who's safety would be paramount to the hotels owners...

 

[nicsin02]

Thanks guys, knew I was right, just wanted it confirmed, so Is it only a TT system that we would use an RCD as fault protection because few months back I was testing food distribution place and 50% of the outside lorry sockets failed Zs test and the customer was adamant that other companies had passed as these circuits were RCD protected?

 

[telectrix]

Dith a TT system, unless you can get sub 1 ohm Ze readings, you'll never meet the Zs figures formost OCPDs and so you have to rely on ther |RCd

 

[Guest112]

Thanks guys, knew I was right, just wanted it confirmed, so Is it only a TT system that we would use an RCD as fault protection because few months back I was testing food distribution place and 50% of the outside lorry sockets failed Zs test and the customer was adamant that other companies had passed as these circuits were RCD protected?

yes the results would pass as rcd protected although I would confirm that the ze from the db subblying the outside sockets was high resulting in a high zs to confirm the circuit isn't producing the high reading

 

[Guest55]

its an inspection , yes ?
then do nothing other than record your measured values in the report , along with any defect code you think is appropriate.
dont start worrying about any necessary changes to get it fixed , sort that when the inspection is completed.
you may find high Zs readings are the least of your worries when testing a hotel. ;-)

 

[somersetsparks]

In this situation with rcd protection on the affected (poorly designed) circuits
It would be the oher factors that would concern me,
I.e volt drop
And obviously these circuit are well over the maximum length values given in BS7671, therefore if there were a short circuit on a cable, could the MCB be relied upon to break the circuit in time?

 

[Risteard]

Whether we like it or not, it is permissible to rely on the RCD protection with a high Zs (or supplementary bonding is another one). However, it would be worth confirming the line/neutral loop impedance as well as the earth fault loop impedance.

 

[i=p/u]

Badly designed, corners cut. C2... Potential hazard if rcd sticks to itself ???

 

[Deleted member 9648]

If it's a TN system at the design stage it would be unacceptable to rely on an RCD for earth fault protection,the circuit must be designed and installed with the max Zs of the OCPD in mind.
If you are carrying out an EICR on an existing install then the only consideration with Zs as far as coding is concerned is compliance with disconnection times,and if there is a working RCD in circuit then disconnection times are met,so no code is applicable.
Other issues which may result from the reason for a high Zs may warrent a code.....but not because of Zs and disconnection times.

 

[Guest55]

i'd give it a C3 for excessive Zs , no danger but requires improvement.

 

[Deleted member 9648]

i'd give it a C3 for excessive Zs , no danger but requires improvement.

In order to code it would have to not comply....if the disconnection time are met it complies.

 

[Guest55]

dont care , my decision is final lol.
;-)